Airborne particle pollution, especially particle matter size less than 2.5 μm diameter range (named “PM2.5”), is a big concern for countries like China, where the speed of industrialization stretches the boundaries of regulatory requirements.
As a consequence of increasing consumer empowerment, the demand for information about the air quality of living spaces is increasing. Especially in China, excessive PM2.5 pollution has become a common problem in the last decade. This problem is also validated by continuous measurements in various Chinese cities. The data is publicly available and can be simultaneously monitored by mobile phone applications or through the web.
Availability of this data as well as continuous national and international media attention has created strong consumer awareness about the problem.
Official outdoor air quality standards define particle matter concentration as mass concentration per unit volume (e.g. μg/m3). The average PM2.5 pollution concentration in mainland China has been calculated based on satellite data, and it has been found that the majority of the country exceeds the World Health Organization limits of 10 μg/m3, with some regions reaching and even exceeding PM2.5 concentrations of 100 μg/m3.
Standardized reference measurement methods are based on measuring the mass of deposited or captured particles per air sampling volume for example using a quartz crystal microbalance, a tapered resonator, an impactor, or weighing filters and sieves.
However, these systems require professional operational guidelines for handling the manual part of the measurement (e.g. weighing a filter and sieve) and/or periodic maintenance for cleaning the accumulated mass, maintaining various system components and calibration.
Resonance based mass sensing for aerosol contamination monitoring has been proposed. For example, use of a micromachined silicon cantilever device with a picogram level of mass resolution for personal exposure monitoring has been proposed. Filters can be used for eliminating large particles and an electrostatic sampler can be provided for depositing nanoparticles on the cantilever. For example WO 2013/064157 discloses a MEMS based resonant particle measurement device, designed for measuring aerosol nanoparticles in an air flow stream.
Mechanical sensors which operate based on resonance operate in a range where the added mass is small compared to the initial resonator mass. However, continuous mass accumulation during the lifetime of the sensor is inevitable. This problem is more pronounced for MEMS scale devices, in which mechanical and/or chemical cleaning of the accumulated mass is not possible—at least for consumer applications. This gives the sensor a limited lifetime.
Optical sensors are also known. These do not suffer from the same drawback of limited lifetime, but they are less able to give information relating to particle mass.
There is therefore a need for a personal mass sensor which can be designed to have a longer lifetime and give accurate mass readings.